Compacted clay liners have been used in municipal and hazardous waste landfills. Typically, soils rich in clay are used for constructing compacted clay liners because soils rich in clay have low hydraulic conductivities. Many regulatory agencies require compacted clay liners in landfills to have hydraulic conductivities less than 10−9 m/s. The hydraulic conductivity of compacted clay liners can vary depending on the composition of the clay, the composition of the soil, water content in the clay, and method of compaction. Low hydraulic conductivities allow the liners to prevent seepage of pollutants out of the landfills. Compacted clay liners are typically formed by spreading a layer of clayey soil and compacting the layer with a roller. Very high pressures are often applied to the soil to ensure that the soil is well compacted so that the hydraulic conductivity is sufficiently low. Heavy compactors weighing over 18,000 kg are often used to compact the soil layers. The compactors can have footed rollers, such as a sheep's foot roller. These rollers have small protruding feet which concentrate the compactive energy of the roller into a small area.
Geosynthetic clay liners are another type of liner used to control seepage out of landfills. Typically, geosynthetic clay liners are made by enclosing bentonite or another expansive clay between two layers of geosynthetic textile. The layers can be bonded together with adhesive or held together by stitching or other fasteners. This forms a blanket-like sheet that can be placed in a landfill as a hydraulic barrier. Sometimes geosynthetic clay liners are used in place of or in addition to a compacted clay liner. Both clay liners can be used in combination with geomembranes and geogrids to provide additional impermeability and structural strength.
Various combinations of soil composition, moisture content, and compaction methods have been used in attempts to minimize the hydraulic conductivity of clay liners. With proper care and maintenance, clay liners can provide low hydraulic conductivities such as below 10−8 m/s or 10−9 m/s. However, clay liners are vulnerable to becoming more permeable in certain situations. Non-homogenous elements, such as soil clods in a compacted clay liner, can increase the hydraulic conductivity of the liner. Additionally, interfaces between layers of clay can create pathways of higher hydraulic conductivity through which fluids can seep. Because moisture content can affect the permeability of the liner, changes in moisture content occurring over time can alter the hydraulic conductivity of the liner. Measures used to prevent drying of clay liners have included limiting the exposure of the liner to atmosphere or periodically spraying the liners with water. However, spraying with water can potentially increase the hydraulic conductivity of the liners if the liners become too wet.